The general switching power supply usually uses an overvoltage protection circuit to control the pulse width modulation (PWM) control circuit in the switching power supply to stop outputting the pulse width modulation signal when the output terminal of the switching power supply has an overvoltage, thereby reducing the output voltage at the output terminal to perform overvoltage protection and protect any one of the internal circuit of the switching power supply and the external system coupled to the output terminal from being damaged.
The output terminal may have overvoltage due to two cases, one of which is caused by the failure of the internal circuit of the switching power supply unit (for example, caused by the failure of a feedback circuit) and the other is caused by the external system (for example, caused by the back electromotive force feedback caused by the external system with a decelerating motor). However, the conventional overvoltage protection circuit cannot distinguish the two cases, so that the conventional overvoltage protection circuit will still control the switching power supply to immediately reduce its output voltage for overvoltage protection when the user normally operates the external system and the back electromotive force feedback is caused, which may cause the external system shut down and cannot be used normally and bother the user.